Magnetic resonance imaging (MRI) diagnostic apparatuses have been put into practical use as a method of examining an internal structure of an object to be measured without injury. At present, many such apparatuses are being operated at general hospitals in local communities and so on, playing an active role on site in medical image diagnostics along with X-ray CT. The MRI exploits the interaction between an atomic nucleus spin (nuclear spin) and an electromagnetic field, which is called as nuclear magnetic resonance (NMR). The resonance energy may be frequencywise converted into a frequency of several tens of megahertz (the frequency band used for FM radio transmission). In the MRI, therefore, the electromagnetic energy irradiated to a test body is fairly low as compared with visible light and X-rays, and is less invasive. However, the low interaction energy for NMR/MRI leads to a related disadvantage, i.e. low detection sensitivity, in theory. In reality, although the MRI diagnostic has become widely used, its resolution is lower than that of X-ray CT. Moreover, since MRI is directed to the hydrogen nucleus (proton, 1H), which has the largest interaction energy among atomic nuclei, it visualizes the density of hydrogen atoms in water, or lipid of a living tissue. Therefore, MRI has found few applications on organs having a low hydrogen density such as the lung. To address such problems, research has been conducted into improving the detection sensitivity of MRI, including application of a very high magnetic field and high coil efficiency. Each attempt at improvement, however, seems to have reached a plateau. In order to achieve an even higher sensitivity, it will be necessary to introduce an innovative technology which studies in depth the principle of NMR. Practical examples of research which has been achieving a success in fulfilling the above described demands include an introduction of a rare gas having a high nuclear spin polarization.
Gases having no sensitivity to MRI at a normal pressure will drastically change the characteristics of MRI when the polarization of nuclear spin of the gases are increased. The nuclear spin polarization of rare gases have been studied for application to the basic science. A method increasing nuclear spin polarization in a rare gas is as follows. The rare gas is filled in a vessel together with an alkali metal such as rubidium or the like. The contents of the vessel are then irradiated with circularly polarized light. Matching the wavelength of the light to the D1 resonance of the alkali metal atom results in a polarization of electron spin of the alkali metal atom due to D1 resonance absorption. The polarized alkali atom collides with the rare gas atom. Through a hyperfine interaction between the electron spin and the rare-gas nuclear spin upon collision, the electron spin polarization is transferred to the nuclear spin polarization. The nuclear spin polarization of the rare gas obtained through this method is very high as compared with a nuclear spin polarization in the conventional MRI. When such a polarization is applied to MRI, the sensitivity will increase by a factor of several tens of thousands to obtain a magnetic resonance signal stronger by not less than 1000 times that of the same volume of water, so that such a rare gas has come to be used for MRI.
In the application to MRI, a polarized rare gas is supplied to a test body in two ways. One way involves a method in which the rare gas is stored together with an alkali metal vapor in a vessel under a static magnetic field. The vessel is irradiated with a laser light to polarize the atomic nucleus of the rare gas through the laser light irradiation, and the rare gas is then supplied to the test body. The other way involves (1) a method in which a mixed gas of the rare gas and the alkali metal vapor is passed through a laser light irradiating part in a static magnetic field to continually polarize the rare gas, and the rare gas is supplied to the test body, or (2) a method using a flow-type polarization vessel conducting the polarization while flowing the mixed gas. The sensitivity of MRI is determined by (1) the energy of the interaction between the nuclear spin and the electromagnetic field, and (2) the product of a square of the nuclear spin polarization and the number of polarized nuclei. This latter product provides a figure of merit for optimization that depends on various parameters. These parameters include, but are not limited to, intensity and circular polarization of laser light in the D1 resonance, a spin exchange rate between the electron spin and the nuclear spin, a spin relaxation time, a pressure of the rare gas, a pressure of nitrogen gas or 4He gas added thereto, and so on. The intensity and circular polarization of the laser light are preferably comparatively high. The spin exchange rate is determined by a combination of a rare gas and an alkali metal and the alkali-metal atomic number density, and the value of the spin exchange rate is preferably comparatively high. The alkali atomic number density is dependent on a temperature. The spin relaxation times of the electron and the nucleus depend on the alkali atomic number density, gas pressure, vessel wall and so on, in which the relaxation time is preferably comparatively long. Taking into consideration all of the above, conditions most suitable for a polarization vessel are as follows:
(1) the light incident part of the vessel should not impair the intensity of the laser light generating D1 resonance;
(2) a more completely circularly polarized light in the vessel should be realized in order to promote a high nuclear spin polarization;
(3) paramagnetic impurities should be decreased on the surface of the vessel as far as possible in order to obtain a long relaxation time;
(4) the vessel should have a high alkali resistance;
(5) the vessel should have a high pressure resistance because a higher gas pressure in the vessel, for example not less than several atmospheric pressure, is preferable; and
(6) when the vessel is used for the polarization of 3He, the vessel should have no permeability for 3He.
Further, when the vessel is applied to basic science, especially when it is used in precision experiments and the like, it is required to have an certain accuracy, so that
(7) the thickness and material of the vessel are preferably uniform; and when the vessel is used in an neutron scattering experiment,
(8) the material of the vessel must have a transparency for neutrons.
JP-A-11-309126 describes an apparatus for producing a polarized rare gas. A flow-type polarization vessel of a coaxial, multi-cylindrical configuration is created by combining outer and inner cylinders, each made of silica glass with a clearance of 0.5 mm. A mixed gas of a rare gas and a vapor of an alkali metal, such as rubidium, is caused to flow in one direction into the clearance. An excited light is irradiated into the flow-type vessel and a magnetic field is applied perpendicularly to the excited light irradiating surface. However, in the apparatus for producing the polarized rare gas according to JP-A-11-309126, since amorphous silica glass is used for both the outer cylinder and inner cylinder constituting the flow-type vessel, circularly polarized laser light will pass through the silica glass from the curved outer cylinder into the clearance to cause the scattering of the laser light, thereby impairing the intensity of the laser light. Moreover, the circular polarization may significantly collapse. The decrease of laser light intensity and the collapse of circularly polarized light will deteriorate the performance of the rare gas polarization. Further, although the vessel must be chemically resistant to a high temperature alkali-metal vapor, the glass vessel used for the above application is less chemically resistant to a high temperature alkali metal. As such, the vessel may deteriorate in a short period of time. When the apparatus is used for 3He gas, there is a problem in that 3He permeates into the glass. Further, since the vessel for rare gas polarization made by glass sculpture cannot have generally uniform thickness and material, there is a problem in the incumbent reduced accuracy when being used for precision experiments in basic science.
JP-A-2003-502132 describes a flow-type polarization vessel of such a configuration that a flat disc-shaped glass window is fitted onto ends of a glass cylinder. When using a diode laser, the oscillation width is significantly smaller than the natural width of the D1 resonance, so that the gas pressure is raised for the purpose of expanding the resonance width by a Doppler effect. Further, a light entrance window is made flat for the purpose of preventing the intensity reduction of the laser light. However, the light entrance window of the flat glass plate cannot obtain the required pressure resistance unless it has a sufficient thickness. Actually, in the polarization vessel according to JP-A-2003-502132, a thick disc having a thickness of not less than 5 mm is used in the light entrance window with a diameter of 24 mm. In the polarization of the rare-gas nucleus, it is important that the circular polarization of the incident light is large. JP-A-2003-502132 describes that the window should not have birefringence so as not to impair the circular polarization of laser light. However, when welding such glass, a problem of strain cannot be avoided. Birefringence may occur even at a slight strain, and collapse the circularly polarized light. The effect of birefringence increases in proportion to the window thickness. Further, the high temperature alkali-metal vapor adheres to, and penetrates into, the glass surface to corrode it. The alkali-metal atoms located on the glass surface are not polarized, so that the nuclear spin polarization of the rare gas is significantly depolarized. Also, when the flow-type vessel is used for 3He polarization, it is necessary to increase the spin exchange rate, and hence potassium may be used in addition to rubidium (see, e.g., E. Babcock et al., Phys. Rev. Lett. 91, 123003 (2003)). The problem on the corrosion resistance becomes more serious. Since 3He permeates into glass, this may also cause a problem. And, a vessel using such a thick glass may not be suitable for use in basic science. Additionally, the vessel shown in JP-A-2003-502132 uses a glass material containing boron. Such a glass material absorbs neutrons. As the thickness of the glass is increased, the glass is less likely to allow the passage of neutrons, thus disabling the experiment of neutron scattering.
JP-A-2003-245263 describes a structure having a light entrance window in which quartz or sapphire having an excellent light transmittance is used in a part or entire part of the flow-type polarization vessel. JP-A-2003-245263 merely mentions quartz and sapphire as an example of the material having an excellent light transmittance for the light entrance window, but there is neither any description nor suggestion of the kind of crystal, such as a single crystal and a polycrystal, or the determination of the orientation of crystal axis.